brake pads

this may be a silly question, but how do you know if the brake pads are worn down enough to need new ones? i got my tires rotated a few weeks ago, and the mechanic said they needed replacing. i didnt pay any mind to that, and told him they were only about 2-3 years old. however, i am curious as to wanting to check myself. any advice?

[97 blazer LT 150 k miles 4X4]

You will hear squealing when you are driving.

When you press the brakes, the squealing will go away.

Why not just look at the break pads to see how warn they are. You should be able to tell without actually removing the pads by looking through the wheels. You be able to see how thick the pad is. If you can't see just remove the wheel and look. Sometimes the breaks don't give any warning. I ended up having to replace a rotor when out of the blue the breaks started making noise. No warning squeeks, Just alot of metal rubbing metal and my rotors were toast.

Not all brake pads have the squealer sensor on it. The only way to know for sure is to pull the wheel off and inspect the pads directly, both inner and outer. Sometimes the caliper has to come off also to see clearly.

Check them as mentioned above, they should replaced if there is less than 1/8" of pad left.